1. Field of the Invention
The present invention relates to a fiber optic More particularly, the present invention relates to a fiber optic repeater that provides passive propagation of optical signals as well as the capability of inputting and reading data in the optical signals.
2. Description of the Related Art
Fiber optic communication systems are well known in the prior art. In a fiber optic local area network, for example, optical fibers are connected between a plurality of nodes at which data is read from and input to the optical signal propagated by the optical fibers. Conventional nodes having both reception and transmission capability, however, require active optical repeaters, which must be powered to propagate signals in the optical fiber data bus across the nodes. An electronics failure or power outage will disrupt service in the data bus. Consequently, it is desirable to provide local area network nodes with a repeater that provides continuous throughput of an optical signal even in the event of electronic failure or power outage. Purely passive optical repeaters, however, have the disadvantage of producing a cumulative attenuation that limits the capabilities of a fiber optic communication network.
In certain national security applications, fiber optic communication networks are particularly desirable because optical signals are not affected by electrical radiation that disrupts or produces noise in conventional electrical signals. In addition, optical signals provide greater security against electronic surveillance because they do not generate easily detectable radiation.
One particularly promising national-security application for fiber optic communication systems is in the underwater monitoring of surface ships and submarines. Such systems typically comprise a plurality of listening devices arrayed on the ocean bottom or in sub-surface locations. Transducers in the listening devices convert sound waves generated by marine vessels into electrical signals, which are relayed by submarine cables to surface monitoring stations. In view of the above-described security advantages of optical fibers, it is desirable to convert these electrical signals into optical signals at each node and to use optical fibers as the interconnecting cables between listening devices and the monitoring station. Such systems, however, require optical repeaters at the listening devices that will propagate very low- power optical signals and have a passive-throughput capability to prevent disruption of a system in the event of electronics failure.
One possible optical repeater for use in such underwater monitoring systems is disclosed in U.S. Pat. No. 4,445,751 issued to Divens et al., which is incorporated herein by reference. Divens et al. discloses an optical repeater using an optical waveguide interferometer formed in a lithium niobate substrate. The optical waveguide is formed in the upper surface of the substrate by diffusing titanium into the lithium niobate material. The waveguide divides into two substantially parallel branches to form the interferometer. The waveguide is symmetrical and bidirectional to permit light to propagate in either direction through the repeater. Electrodes positioned between the central branches of the waveguide as well as alongside outer portions of the substrate permit selective modulation of the optical signal to provide the capability of adding data to the optical signal at each mode.
Known interferometric optical repeaters, however, do not provide the capability of outputting data from the optical signal at the node. Such two-way communications is desirable in many communication networks, for example, to reprogram the undersea listening devices described above.